Entamoeba Histolytica Testing and Parasitic Investigation
of Stool Specimens Algorithm
Mayo Clinic’s Parasitology Laboratory recommends the Parasitic Investigation of Stool Specimens testing algorithm to guide the selection of diagnostic parasitology tests based on the patient’s presenting symptoms, environmental exposures, and immune status. The traditional method for identification of parasites in stool is the ova and parasite examination (#50016 Parasitic Examination), involving a subjective microscopic examination by highly trained and experienced technologists. Although appropriate for identifying many exotic parasites, the method is less sensitive for detecting the most common parasites in the United States, Giardia intestinalis (also known as Giardia lamblia or Giardia duodenalis) and Cryptosporidia spp. These parasites may be suspected when patients present with watery diarrhea and have a supportive history of exposure. In this setting, an enzyme-linked immunosorbent assay (ELISA) for Cryptosporidium spp (#80335 Cryptosporidium Antigen, Feces) or Giardia lamblia (#80231 Giardia Antigen, Feces) is a more timely and cost-effective initial test. For patients who reside in or have visited a developing country or area where helminth infections have been reported, microscopic examination of feces, performed by the traditional ova and parasite method (#50016 Parasitic Examination), is an appropriate first step in diagnosis.
Entamoeba histolytica may occasionally be considered in the differential diagnosis. Although infection with Entamoeba histolytica is rarely encountered in countries with adequate water and sanitary conditions, travelers to and immigrants from endemic areas may be at risk for infection. While the diagnosis may be suspected by the traditional stool ova and parasite microscopic examination, the test does not permit differentiation of Entamoeba histolytica from the nonpathogenic Entamoeba dispar. The test of choice for detecting Entamoeba histolytica is #84042 Entamoeba histolytica Antigen, Feces. This ELISA-based test is more sensitive than microscopic examination of feces and is specific for Entamoeba histolytica.
When disseminated disease is suspected, the assay for serum anti-Entamoeba histolytica antibodies, #9049 Entamoeba histolytica Antibody, Serum, may also be performed. This assay is particularly useful in supporting the diagnosis of amebic liver abscess in patients who do not have a definite history of intestinal amebiasis and the stool antigen assay is negative. It is most helpful for evaluating patients who have not spent substantial time in endemic areas and, thus, would not be expected to have antibodies from a previous infection.
Authored by Pritt BS, Plumhoff EA, Dale JD